What does it mean when the heater is turned on with A/C?

[StephanieRose]

Turning on the heater with A/C means the car's compressor is running, which will additionally increase the vehicle's fuel consumption. Reasons why turning on the heater with A/C increases fuel consumption: In A/C mode, since the car's air conditioning compressor is always in operation, it will inevitably lead to increased fuel consumption and additional engine load. When using the heater, the A/C switch should be turned off to prevent the compressor from running and causing extra fuel consumption during heating. Function of A/C: A/C is the abbreviation for air conditioning, with its full name being air condition, which can be understood as air control or air adjustment. It achieves the purpose of cooling or heating through different operation modes of the air conditioning compressor, hence it has dual meanings of cooling and heating. However, in cars, since the heating function of the vehicle's air conditioning is accomplished through the heat from the engine's cooling water and the heating wires on the glass, A/C only has the single meaning of cooling in this context.

[ConnorLee]

Turning on the A/C button while using the heater means activating the air conditioning compressor during heating mode. Having driven several car models, I find this trick particularly useful. The primary purpose is dehumidification and defogging—for instance, when driving in winter or rainy conditions, if the windshield fogs up, relying solely on the heater takes a long time to clear. But with the A/C button engaged, the compressor and blower dry the air rapidly, clearing fog almost instantly while improving cabin comfort. Of course, running the compressor slightly increases fuel consumption, so it’s unnecessary to keep it on continuously. You can manually control it or let an automatic climate system adjust temperatures for switching. My tip: turn it on for a few minutes during heavy fogging, then deactivate A/C once cleared—this saves fuel and enhances safety. Experiment with your car’s system, as brands differ (e.g., Japanese cars defog faster).

[AllieFitz]

As a car enthusiast, I often tinker with in-car systems. Turning on the A/C with the heater means running both heating and cooling functions simultaneously. The A/C controls the compressor for cooling and dehumidification. The compressor removes moisture from the air, working in tandem with the heated airflow to quickly dry and warm up the cabin. This is particularly effective on highly humid days, such as when driving with fogged-up windows after rain—turning on the A/C can instantly restore visibility. For automatic climate control, pressing the heater button may prompt the system to automatically activate the A/C to assist with dehumidification. However, it's important to note that keeping the compressor constantly running can increase engine load by about 5-10%, affecting fuel efficiency, though short-term use is fine. Personally, I recommend setting a higher target temperature when adjusting the climate control, as the system will intelligently balance the dehumidification effect. Different cars, such as German or American models, have varying defogging logic—just a few minutes of use will show the results.

[VonNicole]

In my daily driving routine, I usually operate like this: when turning on the heating, I press the A/C button to assist in defogging and dehumidifying. The A/C makes the air conditioning system more efficient at removing moisture, especially in winter or rainy days when the cabin is humid and visibility is poor—it works much faster this way. The operation is simple: press the heating button first, then the A/C button, and the fog will disappear in seconds. If I don’t turn off the A/C, fuel consumption will increase slightly, but the impact isn’t significant. Automatic air conditioning is even more convenient—just set the temperature, and it adjusts itself. This makes driving safer and more comfortable.